1. Field of the Invention
The present invention relates to a fire alarming system.
2. Description of the Related Art
In a conventional fire detection system, a plurality of terminal devices such as a fire sensor, a transmitter, a manual box, or other devices to be controlled are connected to a fire alarming signal receiving device, and each terminal device is assigned its own address. The fire alarming signal receiving device calls terminal devices one after another according to the addresses. At a time, only the called terminal device can transmit information about itself (for example a fire alarming signal) to the fire alarming signal receiving device. This technique is known as polling. In this technique, a plurality of alarm signal lines are connected to the fire alarming signal receiver wherein a plurality of terminal devices are connected to each alarm signal line, and thus a great number of terminal devices are connected to the fire alarming signal receiver. The fire alarming signal receiving device calls these terminal devices one after another via a data processor provided in the fire alarming signal receiving device and acquires information about for example an event of a fire from each terminal device.
In the polling technique, however, any terminal device cannot send information to the fire alarming signal receiving device until it is called by the fire alarming signal receiving device. The calling period increases with the increase in the number of terminal devices connected to the alarm signal lines. If the terminal devices include such a device which should transmit information to the fire alarming signal receiving device in a predetermined time period after the event of information at that device, the maximum number of terminal devices which are allowed to be connected to the signal lines is limited by the above predetermined time period.
A specific example of a terminal device which should transmit information in a limited time period is a manual box. When a fire occurs, if one presses a button or the like of a manual box, the manual box transmits an alarm signal to the fire alarming signal receiving device. The fire alarming signal receiving device transmits a response signal to the manual box thereby indicating that the fire alarming signal receiving device has received the alarm signal. If the manual box receives the response signal, the manual box turns on a response lamp or the like provided on the manual box thereby indicating that the alarm signal was generated by that manual box.
In the worst case, the time required for the response lamp of the manual box to be turned on after the manual box was activated becomes as long as one polling period. If the waiting time from the activation of the manual box to the turning-on of the response lamp is so long, the person, who activated the manual box, will wonder if the fire alarming system is in correct operation. To reduce the waiting time to a desirable level, it is required that the number of terminal devices connected to one signal line be limited to a not large value.
One known technique to ease the above limitation is as follows. A fire alarming signal receiving device sequentially calls a plurality of terminal devices connected to signal lines extending from the fire alarming signal receiving device and acquires information about the status of each terminal device wherein a higher transmission priority is given to the particular information of terminal devices having a manual box and the information of these terminal devices is transmitted before other information. If the fire alarming signal receiving device receives the particular kind of information from a manual box, fire alarming signal receiving device preferentially calls those terminal devices having a manual box before calling other terminal devices and determines which manual box has been activated.
In the conventional technique, as described above, when the fire alarming signal receiving device receives the particular kind of information from a terminal device having a manual box, the fire alarming signal receiving device calls the terminal devices having a manual box one by one to determine which manual box has been activated. Therefore, the fire alarming signal receiving device still requires a long time to determine which terminal device having a manual box has been activated, and thus the terminal device still requires a long time to turn on a lamp for indicating that the fire alarm signal has arrived at the fire alarming signal receiving device, in particular when there are a great number of terminal devices having a manual box.
One possible technique to solve the above problem is to call at the same time all terminal devices having a manual box instead of calling them one by one when the fire alarming signal receiving device receives the particular kind of information from a terminal device having a manual box. In response to the call from the fire alarming signal receiving device, the terminal device whose manual box is in an activated state transmits the signal to the fire alarming signal receiving device. Thus, it is possible to reduce the time from when the manual box is activated until the terminal device turns on the lamp to indicate that the fire alarm signal has arrived at the fire alarming signal receiving device.
However, the above modification leads to another problem. That is, if a plurality of manual boxs are activated at substantially the same time, the terminal devices having these manual boxs respond at the same time. As a result, signals collide with each other, and the fire alarming signal receiving device cannot determine correctly which manual box have been activated.
Furthermore, in the conventional polling technique, the data transmission rate over the alarm signal line should be high enough to acquire the information from a large number of terminal devices in a limited time period as short as for example 5 sec. Therefore, the transmission line is required to have a large number of parallel bit lines, which result in an increase in cost.
In a known technique proposed to solve the above problem, the plurality of terminal devices are divided into groups and an idle period is provided between signals calling the adjacent groups so that in the idle period a terminal device can transmit an interrupt signal to the fire alarming signal receiving device wherein it is possible to determine which group the terminal device belongs to from the interrupt signal. If the fire alarming signal receiving device receives an interrupt signal, the fire alarming signal receiving device calls, one by one, preferentially the terminal devices belonging to the group determined from the interrupt signal.
However, in this polling technique in which if a terminal device detects an event of a fire, it transmits in an idle period to the fire alarming signal receiving device an interrupt signal with information (such as a group number) indicating which group the terminal device belongs to, and the fire alarming signal receiving device calls the terminal devices in that group one by one, still a long time is required to determine which terminal device has detected the event of the fire, in particular when each group includes a large number of terminal devices.
A fire alarm/security system is also known in which a fire alarming signal receiving device polls terminal devices such as a fire sensor, a smoke exhausting system, a fire extinguishing system, and a burglar alarm system, or transmitters connected to these terminal devices, and the called terminal devices transmit the information representing the status to the fire alarming signal receiving device whereby the fire alarming signal receiving device acquires the monitored information. In this fire alarm/security system, control information is transmitted to the terminal devices called by the fire alarming signal receiving device.
In this type of fire alarm/security system, there is a possibility that an error occurs during the transmission between the fire alarming signal receiving device and the terminal devices. For example, there is a possibility that an area where no fire occurs is regarded as having a fire in error. Another possibility is that a terminal device concludes in error that it has received a control command from the fire alarming signal receiving device and the terminal device activates a smoke exhausting system or a fire extinguishing system.
In the conventional fire alarm/security system, to avoid the above problem arising from the error in the transmission between the fire alarming signal receiving device and the terminal devices, the fire alarming signal receiving device generates a primary sum code by adding together an address code and a command code, and transmits the resultant primary sum code to the terminal devices. The terminal devices produce a first sum code by adding the address code and the command code received from the fire alarming signal receiving device. If the obtained first sum code is equal to the received primary sum code, the terminal device regards the signal received from the fire alarming signal receiving device as valid. On the other hand, each terminal device calculates the sum of the returning-back data, the address code, the command code, and the primary sum code thereby producing a secondary sum code, and transmits the resultant secondary sum code together with the returning-back data, address code, and command code to the fire alarming signal receiving device. The fire alarming signal receiving device calculates the sum of the returning-back data, the address code, and the command code received from the terminal device as well as the primary sum code produced by the fire alarming signal receiving device thereby producing a second sum code. If the obtained second sum code is equal to the received secondary sum code, the fire alarming signal receiving device concludes that the terminal device has correctly received the address code and the command code transmitted by the fire alarming signal receiving device.
In the above conventional system, each terminal device has a signal transmission/reception circuit (including a parallel-to-serial converter). If an active bit is input to the signal transmission/reception circuit, it is converted by the circuit into a low-level signal and output over the transmission line. On the other hand, if an inactive bit is input to the signal transmission/reception circuit, it is converted by the circuit into a high-level signal and output over the transmission line. If a collision occurs between a low-level signal and a high-level signal on the transmission line, the result is a high-level signal.
If the fire alarming signal receiving device receives a low-level signal via the transmission line, the received low-level signal is converted into an active bit by a signal transmission/reception circuit (including a parallel-to-serial converter) provided in the fire alarming signal receiving device. On the other hand, if the fire alarming signal receiving device receives a high-level signal via the transmission line, the received high-level signal is converted into an inactive bit by the signal transmission/reception circuit in the fire alarming signal receiving device. Therefore, if a collision occurs between a low-level signal and a high-level signal on the transmission line, the fire alarming signal receiving device receives the high-level signal. The received high-level signal is converted into an inactive bit by the signal transmission/reception circuit provided in the fire alarming signal receiving device and acquired into the fire alarming signal receiving device. In the above conventional system, as described above, if an active bit and an inactive bit are transmitted at the same time over the transmission line from different terminal devices such as fire sensors, the inactive bit is acquired into the fire alarming signal receiving device.
In such the fire alarm/security system according to the conventional technique, if an equal address is assigned by mistake to two different fire sensors, and if one of the two fire sensors transmits a fire alarm signal as the returning-back data while the other fire sensor having the same address transmits information (non-fire signal) indicating that the fire sensor is in a normal status, the above fire alarm signal cannot arrive at the fire alarming signal receiving device. Another possible problem is that when an equal address is assigned by mistake to two different fire sensors, the second sum code can be equal to the secondary sum code, and thus it is impossible to detect the event of the above transmission error.
For example, when the address AD and the commands CM1 and CM2 are FFh, 01 h, and 00 h, respectively, in hexadecimal form (11111111, 00000001, and 00000000 in the form of 8-bit binary numbers; each hexadecimal number is represented with an "h" at its end), if one of the two fire sensors having the same address transmits returning-back data D1 having a value of 01 h (00000001 in the form of an 8-bit binary number) indicating that a fire occurs while the other fire sensor having the same address transmits returning-back data D2 having a value of 00 h (00000000 in the form of an 8-bit binary number) indicating the normal status, then the result will be as follows. The primary sum code PS is given as 00 h as a result of the calculation described above. The secondary sum code SS1 for the returning-back data D1 indicating the event of a fire becomes 01 h. The secondary sum code SS2 for the returning-back data D2 indicating the normal status becomes 00 h. Because the same address is assigned to the two terminal devices, the returning-back data D1 and D2 are transmitted at the same time, and the secondary sum codes SS1 and SS2 are also transmitted at the same time.
As a result, collision occurs on the signal line L between the returning-back data D1 and D2. Similarly, the secondary codes SS1 and SS2 collide with each other on the signal line. When a collision occurs between 01 h and 00 h, only the signal corresponding to 00 h can propagate along the signal line for the following reason. Bits having a value of 0 become high on the transmission line while those bits having a value of 1 become low on the transmission line. If a low-level signal and a high-level signal are applied to the signal line at the same time, the resultant signal level on the signal line becomes high. Therefore if 01 h and 00 h, which can be represented as 00000001 and 00000000, respectively, in the form of 8-bit binary numbers, are applied to the signal line at the same time, the collision between a "0"-bit and a "1"-bit results in a "0"-bit on the signal line, and thus only the 00 h signal can exist on the signal line. As a result it seems that 00 h is transmitted for both the returning-back data and the secondary sum code. Therefore, the fire alarming signal receiving device regards the received returning-back data as 0 h, and calculates the second sum code as 00 h according to the calculation rule described above. Thus the calculated second sum code becomes equal to the received apparent secondary sum code. Thus the fire alarm signal cannot arrive at the fire alarming signal receiving device. This is a serious problem in the conventional system. Besides, the fact that such the error has occurred during the transmission cannot be detected.
Furthermore, in the polling operation according to the conventional technique, if the fire alarming signal receiving device detects a transmission error for a terminal device or a group containing that terminal device, the fire alarming signal receiving device stops polling other terminal devices or groups including other terminal devices, and polls again the terminal device which has encountered the transmission error or the group including that terminal device. If a transmission error is detected again in that polling operation, the fire alarming signal receiving device concludes that a transmission error has really occurred and makes an indication of the event of the transmission error.
In the above conventional technique, however, the repetition of the polling operation required for confirming a transmission error causes a delay in the operation of acquiring status information from other terminal devices or the group including that terminal device. This causes an increase in the total time required to poll the terminal devices in the fire alarm/security system.